(a) Field of the Invention
This invention concerns a circuit for protecting an indicator of either the presence of electrical power to a device, and/or whether the ground circuit protecting the device is functional.
It is known in a standard three-wire power supply system, to provide a ground connection for the safe operation of an electrical device. In the past, it has been common practice to periodically check the device's connection with the power supply, as well as inspect the ground circuit to insure the device is properly and safely connected to the power supply. Routine inspections are time consuming and no guarantee against the power supply connections or ground continuity being broken between inspections.
It is further known to associate an indicator with the electrical device for the purpose of indicating whether the device is connected to an active power supply, and/or properly grounded.
Electrical devices such as power cords incorporating an indicator are required to meet or exceed minimum performance and safety standards established by Underwriter's Laboratory (UL). UL standards pertaining to power cords mandate an excessive test voltage (either 1500 VAC or 2150 VDC) be passed through the power cord for one second in order to determine the heat and fire resistant characteristics of the power cord. In the course of the excess Voltage test, the aforementioned indicators become overloaded and "burn-out ".
(b) Description of Related Art
U.S. Pat. No. 4,671,597 to Grill discloses an electrical receptacle having a power indicating light. U.S. Pat. No. 4,606,597 to Bielefeld discloses an electrical connector having an indicator lamp for indicating the availability of electrical current through the connector. U.S. Patent No. 4,520,239 to Schwartz discloses an electrical cord reel and storage system which includes an indicator light which is illuminated when connected to a source of electrical power. U.S. Patent No. 3,924,914 to Banner discloses an electrical safety grounding device including a fuse, a power indicator, and/or a ground continuity indicator. U.S. Pat. No. 3,753,261 (Thaxton), U.S. Pat. No. 3,588,861 (Meyer) and U.S. Pat. No. 3,559,200 (Walters) disclose similar devices to that of Banner. U.S. Pat. No. 3,171,113 to McNamara discloses a modified plug connector having a ground continuity indicator and which is further capable of adapting to either two or three wire systems. U.S. Pat. No. 2,731,629 to Siderman discloses a wiring assembly similar to the device of Grill. U.S. Pat. No. 2,503,677 to McHenry et al. discloses a translucent light bulb socket having a lamp for indicating the location of a defective bulb. U.S. Pat. No. 2,474,407 to Todd et al. discloses a modified plug connector having a polarity indicator for a two wire system. U.S. Pat. No. 2,449,150 to Schnoll discloses a power indicating device similar to that of Thaxton. U.S. Pat. No. 2,112,137 (Brach), U.S. Pat. No. 1,935,771 (Feurstein et al.), U.S. Pat. No. 1,720,463 (Both) and U.S. Pat. No. 1,665,987 (Smith) disclose modified connecting apparatus having power indicators. Canadian Patent 717,200 to Roll et al. discloses an electrical plug connector having an indicating light, however, the device is for a two wire, non-grounded system. In each case, these devices fail to incorporate a circuit protecting their indicator against failure as a consequence of undergoing the aforementioned UL excessive voltage test.